Direct composite canvas background if possible.

This avoids allocating backing for the containter GraphicsLayer.
If the background is opaque, canvas texture can be drawn with
GL blending disabled. If the canvas has global opacity, it also
avoids drawing to an intermediate render-surface.

BUG=236982

Review URL: https://chromiumcodereview.appspot.com/19543014

git-svn-id: svn://svn.chromium.org/blink/trunk@154786 bbb929c8-8fbe-4397-9dbb-9b2b20218538